This invention relates to the art of glass manufacturing using an electrical furnace, and more particularly to a new and improved method and apparatus for controlling the supply of electricity to such furnaces.
When glass is melted electrically, it is very important that a minimum of direct voltage or current be supplied to the glass furnace. This is because d.c. electricity will cause the glass melting electrodes to exhibit a higher than normal amount of wear and may cause undesirable disturbances such as seeds, blisters and hot spots in the glass. These are some of the reasons why a.c. electricity is used to melt glass.
The a.c. electricity supplied to the glass melting electrodes of a furnace is controlled in accordance with actual and desired operating conditions associated with the melting glass batch and electrodes, and such control is effected by devices such as tapped transformers, saturable core reactors and controlled rectifiers in the electrical power supply circuit for the electrodes. Due to the presence of inductance in such circuits, there will be differences in the areas underneath the voltage and current waveforms of the positive and negative half cycles of the a.c. electricity, and this in turn will give rise to a net d.c. component. It therefore would be highly desirable to provide a method and apparatus for supplying controlled a.c. electricity to such furnaces in a manner eliminating or significantly minimizing any d.c. component.